The present invention relates to a new and improved method of providing aerated or foamed plastic surfaces with a relief structure in which a pattern is applied, prior to foaming, to the surface of a base layer which is to be foamed, and during the course of foaming the foaming action is retarded and/or accelerated at certain locations corresponding to the applied pattern, in order to obtain a foamed plastic surface which possesses a relief structure coinciding with the applied pattern. The invention further relates to means for the performance of the aforesaid method.
Attempts have already been made for quite some time to provide aerated or foamed plastic surfaces, possessing a colored pattern or design, for instance materials suitable as floor or wall coverings, with a relief structure corresponding to the pattern. A technique in which initially a pattern is imprinted upon the foam plastic surface and then following the foaming operation there is obtained a permanent compression of the foamed plastic at certain locations of the impressed pattern by the application of external pressure-and heat, is associated with the drawback that it is not readily possible to attain an exact correlation of the printing design with the relief or embossed pattern, and further, that for the purpose of carrying out this technique it is necessary to use precision operating and expensive equipment.
Hence, efforts have been expended for a considerable period of time to dispense with such type pattern-true embossment carried out after the foaming operation.
In U.S. Pat. No. 2,964,799, there is disclosed a method which, among other things, also can be employed to provide an aerated or foamed plastic with a relief-like surface in that during foaming a foamable layer is permitted to travel over a specially constructed heatable roller. This heatable roller possesses raised portions so that only at the locations of the raised portions of the heatable roller is the foamable layer heated to the requisite foaming temperature. Consequently, only at certain locations of the throughpassing web is the foamable layer foamed, and thus there is obtained a relief-like structure for the final product. If it is desired to produce a foamed plastic surface provided with a pattern according to this technique, and wherein there should be realized an exact matching or coincidence between the color pattern and the relief, then the same difficulties arise as with the technique where an originally uniformly foamed material is subsequently embossed.
U.S. Pat. No. 2,961,332, discloses a technique for the fabrication of a relief-like foamed plastic wherein a pattern is impressed at the rear face of a carrier material and certain portions of such pattern are imprinted at the rear face with two foamable thermoplastic resin mixtures. Both of these thermoplastic resin mixtures are notably different from one another by virtue of their content of foaming agent. Consequently, during foaming there is attained a different foaming effect for both resin mixtures and thus a relief-like structure of the final product. It should be readily appreciated that when working with this process it is only possible to fabricate relief patterns of very simple design and in this case there likewise are present the difficulties arising with the previously mentioned methods as regards coincidence between the printing pattern or design and the relief print or pattern.
Furthermore, in U.S. Pat. No. 2,825,282, there is disclosed a method in which a certain dye or ink is applied to a foamable base material. Thereafter, the base material is subjected to radiation so that only at those locations which are coated with the dye does there occur an absorption of the radiation which is sufficient for foaming the base material. With this procedure it is possible to relatively easily attain a good matching or coincidence between the relief print and the printing pattern or design, but such technique however is only capable of being successfully employed with extremely thin foamable layers, namely layers having thickness of about 0.5 mm. Moreover, in order to obtain an appropriate differentiation in the radiation absorption, as a practical matter, there only can be employed a black dye or ink.
Up to now, the best results could be obtained with the practice of the process disclosed in German Patent publication No. 1,504,106, corresponding to U.S. Pat. No. 3,293,094. In this patent, there is disclosed a process wherein a chemical inhibitor is applied in sections to the surface of a foamable layer. This inhibitor varies the decomposition of the foaming agent at these sections, so that after the foaming action triggered by heating, those locations of the surface where the inhibitor has been applied are depressed in relation to the remaining surfaces. With this technique it is possible to add an inhibitor to the dyes or inks employed for printing of the pattern, wherein if desired different dyes can contain different concentrations of inhibitor. Furthermore, it is possible to not directly apply the inhibitor to the surface of the material which is to be foamed, rather to a layer of a resin mixture free of a foaming agent and located over the foamable mass, whereby then the outer surface of the finished material carries the printing design or pattern. With this embodiment the printing design is thus subjected to a pronounced mechanical load and can be particularly easily scratched away at the raised locations. Furthermore, in FIGS. 10 to 13 of this German patent publication there is also disclosed a technique in which a pattern can be imprinted by means of printing dyes upon a thin transfer paper and thereafter there can be applied a foamable layer to the printing design or pattern located at the paper. Then the thus coated paper carrier with the foamable layer located thereon can be heated to the foaming temperature, whereby the foamable layer remains adhering to the paper carrier during foaming and the free surfaces of the foamable material which do not possess any paper carrier exhibit a depression at those locations where the paper carrier has been provided with the inhibitor. After the foaming operation, the paper carrier is removed. This modified process as described in the aforementioned German patent publication also only can be employed where it is desired to provide an aerated or foamed plastic with a desired relief or embossed pattern; it is however not possible to simultaneously apply a colored pattern corresponding to the relief pattern, because this colored pattern, if there is applied to the surface of the paper colored inhibitor-containing inks, would be located at the non-embossed side of the surface.
If the method described in German Patent publication No. 1,504,106 is carried out such that a colored pattern is imprinted upon the foamable base layer, and wherein the employed dyes or inks possess different content of foam inhibitors or are completely free from the same, then it is possible to obtain a very good coincidence between the printing pattern or design and the relief print. The essential drawback of such technique, however, resides in the fact that the surface of the gelled foamable layer only can be printed or impressed with difficulty, and that for imprinting such type relatively thick layers it is necessary to employ high roller pressures and therefore expensive pressure rollers and printing presses.